Field of the Invention
The present invention relates to an edge detection apparatus and an edge detection method.
Description of the Related Art
Recently the use of an image display apparatus that can display high resolution images, the resolution of which is higher than conventional apparatuses, is spreading. Among high resolution images, an image, of which the number of pixels in the horizontal direction is about 4000 (e.g. an image having 4096 pixels in the horizontal direction×2160 pixels in the vertical direction) is called a “4 k image” in general. If the resolution of the input image data to be input to the image display apparatus is different from the screen resolution of the image display apparatus, the resolution of the input image data must be converted into a resolution that is the same as the screen resolution of the image display apparatus.
An example of image processing to convert the resolution of the image data is scaler processing. Scaler processing is image processing to convert the resolution of the input image data by changing (enlarging or reducing) the image size of the input image data. If this image processing is performed using a common method (e.g. bicubic method), jaggies are generated in an image region on a diagonal edge (edge in the diagonal direction), and image quality drops considerably.
A technique to suppress the generation of jaggies is disclosed in Japanese Patent Application Laid-Open No. 2006-33647. In the technique disclosed in Japanese Patent Application Laid-Open No. 2006-33647, a diagonal edge is detected in an image, and for an image region of the diagonal edge, an interpolated pixel is generated using a pixel corresponding to the inclination angle of the diagonal edge (direction of the inclined edge). Further, in the technique disclosed in Japanese Patent Application Laid-Open No. 2006-33647, in a case where a correlation value indicating the presence of a diagonal edge is acquired for a position close to a position of interest and a position distant from the position of interest respectively, a diagonal edge is detected in the position of interest using the correlation value corresponding to the position close to the position of interest.
Here, if a correlation value corresponding to a position close to the position of interest is used with priority, an error occurs in the detection of the diagonal edge. For example, a diagonal line at a small angle, close to the horizontal line, included in a natural image, is a horizontal line in a case where the diagonal line is viewed locally; therefore this diagonal line (diagonal line at a shallow angle, close to the horizontal line, included in a natural image) and the actual horizontal line cannot be distinguished from each other, and this diagonal line is detected as the horizontal line by mistake. As a result, the generation of jaggies cannot be suppressed in the image region of this diagonal line.
Even in the case of a method which does not assign priority to the correlation value corresponding to a position close to the position of interest, a diagonal edge detection error could occur. For example, a correlation value indicating the presence of the diagonal edge may be detected by mistake, as a correlation value corresponding to a position of a character that is different from a character constituting the diagonal edge in the position of interest. As a result, an incorrect diagonal edge, based on an incorrect correlation value, is detected, and major image quality deterioration (image quality disturbance) is generated in the image region where the incorrect diagonal edge is detected.